Hypertension is a serious risk factor for myocardial infarction, heart failure, vascular disease, stroke, and renal failure. The renin-angiotensin-aldosterone system (RAAS) plays an important role in the regulation of blood pressure. Inappropriate increase of aldosterone causes age-related increase in blood pressure and other cardiovascular problems. Aldosterone is synthesized in the ZG of the adrenal cortex and aldosterone synthase is the rate limiting enzyme in its biosynthesis. Aldosterone synthase is coded by Cyp11B2 gene and the human gene has a T/C polymorphism located at -344 in its promoter. Epidemiological studies have suggested that variant -344T of Cyp11B2 gene is associated with hypertension and myocardial hypertrophy. Human Cyp11B2 gene has three SNPs in 1 Kb of its promoter that are in almost complete linkage dis-equilibrium. These SNPs are rs1799998 (T/C at -344), rs10087214 (C/T at -470), rs28659182 (C/A at -663). Thus variant -344T almost always occurs with variants -470C, -663A (named haplotype-I, and variant -344C almost always occurs with variants, - 470T, -663T (named haplotype-II). We have generated transgenic mice by knocking in hCyp11B2 gene containing either haplotype-I or haplotype-II at the HPRT locus. Transgenic mice containing haplotype-I have increased hCyp11B2 mRNA level in the adrenals and kidneys as compared to transgenic animals containing haplotype-II. In addition, male transgenic animals containing haplotype-I have increased blood pressure as compared to transgenic animals containing haplotype-II of hCyp11B2 gene. High salt (4% NaCl diet) and angiotensin-II administration increases blood pressure in transgenic mice containing haplotype-I of hCyp11B2 gene as compared to haplotype-II. Therefore, our hypothesis is that transcription factors bind strongly to the nucleotide sequence of hCyp11B2 gene containing haplotype-I as compared to haplotype-II, and increase its expression. This increases tissue or plasma aldosterone level in human subjects containing haplotype-I. The long term consequence of altered regulation of aldosterone production leads to an increase in blood pressure and cardiovascular complications in human subjects containing -344T allele as compared to -344C allele. These transgenic mice will be used to understand the role of these haplotypes on blood pressure regulation in male and female animals. Transgenic mice may also be used to develop new therapeutic reagents to reduce blood pressure and cardiovascular complications.